Water is used as a rinse in processes that employ photoresist material, such as the fabrication of printed circuit boards and in other industries. A typical sequence in a process for making multilayered circuit boards begins by cleaning and scrubbing a circuit board. Photoresist is applied to the board as a film or liquid. A polymeric photoresist material typically comprises a polymeric film comprised of soluble high boiling point acrylic monomers, fillers, solvents and other adjuvants. The photoresist polymer is photosensitive so that when irradiated with light of the proper wavelength, the polymer becomes soluble in a dilute alkaline solution. The polymer is extremely insoluble in an acidic solution and highly soluble in strongly alkaline solutions. Because of these properties, polymeric photoresist materials are used to cover areas of circuit boards to prevent them from receiving certain coatings.
Light having a certain wavelength (photoimage) is applied to the photoresist to form a photoresist pattern on the board. A developer is then applied and the board is rinsed. A cupric or ammoniacal etchant is applied to remove copper from the board. A stripper then removes the photoresist material and rinse water is applied to the board. The stripper typically may include monoethanolamine, sodium hydroxide and sodium carbonate. The stripper may remove the photoresist polymer in "sheets" and also by converting the polymer to its monomer form. In addition to the soluble monomer, soluble polymer fragments, e.g., 5-7 monomer units long, may be in the rinse. The developer and stripper rinses are subjected to final neutralization and discharged. The etchant material has a high copper content and is treated separately.
Numerous layers having patterned electrodes prepared as above are stacked. The process may include applying an oxide coating, laminating, drilling, deburring and desmearing. Electroless copper plating is conducted. Photoresist material is then applied to the upper and lower surfaces of the stacked patterned layers and subjected to a photoimage. The photoresist is developed and rinse water is applied to the board. Pattern plating applies more copper. The photoresist is stripped and rinsed. A cupric or ammoniacal etchant is applied. A solder strip step is conducted and the board is washed. A solder mask is applied to protect the copper. A developer and rinse water are then applied to the board. Then a solder coat and hot air level occur. The board is then rinsed and dried.
It is difficult to filter the rinse water of the photoresist process. The rinse water contains high concentrations of suspended solids, including photoresist particles. There is also a high biological oxygen demand (BOD) and chemical oxygen demand (COD) from the organic constituents. In view of various contaminants, the rinse water is typically treated for pH adjustment and discharged to a public operated treatment works (POTW). The total volume of rinse water may amount to as much as 40% of the total discharge from manufacturing operations.